The present invention relates to a system for the suppression of noise and its variations for the detection of a pure signal in a measured noisy discrete signal. This system evaluates the information supplied by the measured signal. This information leads to a false detection probability, i.e. a probability of detecting a pure signal where there is not one. The information can be compared with a threshold if it is solely wished to detect the presence or absence of a pure signal, said threshold corresponding to a predetermined false detection rate. The information can also be evaluated if it is wished to known the degree of certainty of the existence of a pure signal.
The invention applies to the detection of discrete signals, i.e. signals produced by intermittent events. The invention more particularly applies to the detection of corpuscular radiation, such as neutron rays or gamma rays.
The detection of a pure signal in a measured noisy signal assumes that it is possible to eliminate the noise in said measured signal. Two cases occur, namely the mean noise level is constant and the mean noise level fluctuates. The invention relates to a detection system when the mean noise level fluctuates. As a constant mean noise level is only a special case of a fluctuating mean noise level, it follows that the system according to the invention can be used for detection purposes when the mean noise level is constant.
The detection of a particular, e.g. nuclear corpuscular radiation takes place in the presence of corpuscular noise having several causes. In the case of nuclear radiation, part of the nose is due to cosmic radiation and the other part is due to the natural radioactivity of the ground and the rocks. There is also noise which is due to the detector used. The cosmic radiation can vary over a period of time and the corpuscular noise due to the rocks and ground is essentially variable, as a function of the place where the measurement is performed. Thus, a particular radiation can only be detected on knowing the noise or at least having an adequate estimate.
The information is stored for a limited time period .DELTA.T. This information storage takes place in a discrete manner. When a particle is detected, the content of a memory is increased by one unit. In general, one or more parameters define the detected nuclear event. In the case where it only depends on a single parameter, the latter can e.g. be the energy. This is the typical case of detecting gamma radiation with the aid of an intrinsic germanium or a NaI scintillator associated with a photomultiplier. This single parameter can also be time, e.g. when measuring the flight time of a particle. The detection condition can also be a complex logic function dependent on the state of several detectors during a very short time interval, generally a few dozen nanoseconds, particularly in the case of detecting particles in a wire chamber.
Usually, to know whether a signal has been detected with the aid of a storage performed for a time interval .DELTA.T, the standard procedure is to evaluate by what value the count obtained exceeds the estimate of the available noise. This noise can in particular be evaluated during an experiment by extrapolating or interpolating the levels of the signal detected in different intervals, e.g. of an energetic type. For this purpose, it is necessary that the counting rate in the intervals where certainty exists that there is no noise is adequate in order to have an estimate of the noise probability density in the intervals where it is wished to detect a particular radiation.
Conventionally, an estimate is made as to whether there is a signal when the counting rate N measured for a period of time .DELTA.T is such that N&gt;B+K..sqroot.B, in which B is the mean estimated noise rate and K is a constant equal to a few units. This known detection criterion is not satisfactory because it is of an empirical nature, the value of K being chosen in a non-deterministic manner on the basis of prior experiments.
An object of the invention is to supply a system for the suppression of noise and its variations for the detection of a pure signal based on a rational criterion deduced from the appearance probability laws of the noise and the expected radiation.
In the prior art, a counting rate is only taken into account if it exceeds the mean noise rate increased by a certain number of times the standard deviation .sqroot.B. However, according to the invention it is considered that whenever the mean noise rate is exceeded it constitutes information concerning the presence or absence of a pure signal. This information is slight when the difference between the counting rate and the mean noise rate or level is low.
The greater the difference between the counting rate and the means noise level, the greater the amount of information supplied by the measurement and the smaller the false detection probability. The practical detection criterion of a pure signal is consequently the obtaining of adequate information, i.e. exceeding a threshold I.sub.m, which is a function of a fixed false detection rate.
The information I supplied by the measurement can be exploited in two ways. It can be simply compared with the threshold I.sub.m to discriminate the presence or absence of a pure signal, whilst it is also possible to measure the difference I-I.sub.m for evaluating the degree of certainty of the presence of a pure signal.
The comparison between I and I.sub.m can take place indirectly on the false detection probability and the false detection level associated with I and I.sub.m. However, the information is preferably used for the exploitation, e.g. by displaying a signal representing the information as a function of the counts.